1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on transfer paper in accordance with electrophotography and, more particularly, to an image forming apparatus which performs development using a two-component developing agent.
2. Description of the Prior Art
As an example of an image forming process of forming an image by electrophotography, there is known a process of forming an electrostatic latent image on an image forming body such as a photosensitive body, developing the formed electrostatic latent image by a developing means to form a toner image on the image forming body, transferring the formed toner image onto transfer paper by a transfer means, and fixing the transferred toner image on the transfer paper by a fixing means to form an image on the transfer paper. Another example is known as a process of transferring a toner image on an image forming body such as a photosensitive body onto an intermediate transfer body serving as an image carrier, transferring the toner image from the intermediate transfer body onto transfer paper by a transfer means, and fixing the transferred toner image on the transfer paper to form an image on the transfer paper.
In the developing step of the above-mentioned conventional image forming process, development using a two-component developing agent containing nonmagnetic toner and a magnetic carrier is often employed, and a developing bias voltage obtained by superposing an AC-bias voltage on a DC-bias voltage is applied.
In development using the two-component developing agent, since only the toner is consumed by development, an appropriate amount of new toner corresponding to the consumed amount must be replenished. Thus, toner replenishment is performed.
Newly replenished toner together with a magnetic carrier is stirred by a stirring means, e.g., a stirring convey screw, a rotary paddle which is like a water wheel, or the like, and mutual friction causes the toner to be charged due to triboelectrification. For this reason, if stirring is not satisfactorily performed, and the toner with charge of less than a predetermined value makes visible an electrostatic latent image, part of the toner is attracted to white portions of an image forming body, i.e., so-called fogging occurs in the image.
Particularly, in an apparatus which employs a toner recycling scheme, recycle toner is often more deteriorated than newly replenished toner and tends to cause the above-mentioned inconvenient phenomenon. When toner having a small particle size or toner manufactured by a polymerization method and having a sharp particle size distribution is used, an image quality (e.g., resolution, tone, and character reproducibility) is high. Therefore, the above inconvenient phenomenon tends to be obvious.
For the image formation conditions of development, transfer, and the like, whether an image is satisfactorily formed substantially depends on the charge amount of toner. However, conventionally, the state of a developing agent is predicted from the use environment, life, and use condition of toner, and the developing conditions and the like are set using a table prepared in advance. A technique is also used for obtaining a suitable image density by changing the developing conditions based on a patch density generated in image adjustment mode. In these methods, the image formation conditions are not set based on the toner charge amount obtained by direct calculation. For this reason, to increase the image density, an image may be developed excessively to cause a problem such as fogging and the like. Particularly, when toner having a small particle size is used, the developing characteristics vary greatly, and when control is performed only by image density detection, an image with a stable image quality cannot be obtained.
It is an object of the present invention to provide an image forming apparatus capable of obtaining a toner charge amount to set optimum image formation conditions based on the obtained toner charge amount.
To achieve the above-mentioned object, according to the first aspect of the present invention, there is provided an image forming apparatus including an image forming body, electrostatic latent image forming means for charging the image forming body to a charging potential Vh (V) by charging means and exposing the image forming body by exposure means to form an electrostatic latent image on the image forming body, developing means for using a two-component developing agent and applying a developing bias voltage obtained by superposing an AC-bias voltage on a DC-bias voltage Vdc (V) to a developing agent carrier to develop the electrostatic latent image formed on the image forming body, thereby forming a toner image on the image forming body, transfer means for transferring the toner image formed on the image forming body onto a recording medium or an intermediate transfer body, cleaning means for cleaning part of the toner image which is not transferred and left on the image forming body, and a controller which controls operation of each of the means, comprising a potential sensor which measures a charging potential on the image forming body, and a patch density sensor which detects a toner attraction amount of a patch image, wherein to perform image formation, the controller calculates a toner charge amount Qt (xcexcC/g) from a potential of a patch image before and after development detected by the potential sensor and an image density of a developed patch image detected by the patch density sensor and sets an image formation condition based on the calculated toner charge amount Qt.
According to the second aspect of the present invention, there is provided an image forming apparatus, wherein setting of the image formation condition according to the first aspect is performed using a table which stores in advance the image formation condition corresponding to-the toner charge amount Qt. According to the third aspect of the present invention, there is provided an image forming apparatus, wherein the image formation condition stored in the table according to the second aspect is a difference (Vhxe2x88x92Vdc) between the charging potential Vh (V) and the DC-bias voltage Vdc (V).
According to the fourth aspect of the present invention, there is provided an image forming apparatus, wherein the image formation condition stored in the table according to the second aspect is a peak value Vacp-p (V) of the AC-bias voltage.
According to the fifth aspect of the present invention, there is provided an image forming apparatus, wherein the image formation condition stored in the table according to the second aspect is a frequency Fac (kHz) of the AC-bias voltage.
According to the sixth aspect of the present invention, there is provided an image forming apparatus, wherein the image formation condition stored in the table according to the second aspect is the DC-bias voltage Vdc (V).
According to the seventh aspect of the present invention, there is provided an image forming apparatus, wherein the image formation condition stored in the table according to the second aspect is a value vs/vp obtained by dividing a peripheral velocity vs (mm/s) of the developing agent carrier by a peripheral velocity vp (mm/s) of the image forming body.
According to the eighth aspect of the present invention, there is provided-an image forming apparatus, wherein the image formation condition stored in the table according to the second aspect is a transfer current Itr (A) used when transferring the toner image onto the recording medium or the immediate transfer body.
According to the ninth aspect of the present invention, there is provided an image forming apparatus, wherein setting of the image formation condition according to the first aspect is performed in image adjustment mode.
According to the 10th aspect of the present invention, there is provided an image forming apparatus, wherein the image formation condition according to the first aspect comprises a plurality of different image formation conditions corresponding to the toner charge amount Qt, and the plurality of image formation conditions are simultaneously set using a plurality of tables which store in advance the plurality of image formation conditions, respectively.
According to the 11th aspect of the present invention, there is provided an image forming apparatus, wherein setting of the image formation condition according to the second or 10th aspect is performed in image adjustment mode.
According to the 12th aspect of the present invention, there is provided an image forming apparatus according to the first or second aspect, wherein the two-component developing agent comprises a magnetic carrier and nonmagnetic polymerized toner having a volume average particle size of 3 xcexcm to 6.5 xcexcm.
As can be seen from the above-mentioned aspects, according to an image forming apparatus of the present invention, the toner charge amount is obtained, and the image formation conditions are set based on the obtained image formation conditions, unlike a conventional method of setting the image formation conditions. For this reason, more suitable development conditions or transfer conditions are set compared to conventional apparatuses, and thus sharp, satisfactory images can be obtained.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.